1. Field of the Invention
The present invention relates to an electrophotographic photoconductor (may be hereinafter referred to as “photoconductor”, “image bearing member”, or “latent electrostatic image bearing member”) which has very high abrasion resistance even when repeatedly used and is capable of forming high-quality images involving less image defects over a long period of time, an image forming method using the electrophotographic photoconductor, an image forming apparatus using the electrophotographic photoconductor, and a process cartridge using the electrophotographic photoconductor.
2. Description of the Related Art
Recently, there is remarkable progress in development of information processing system machines using an electrophotographic process. In particular, laser printers and digital copiers each configured to convert information to digital signals with use of a laser beam to thereby record the information have been remarkably improved in print-quality and reliability. An integration of the technology and speeding-up in recording has been used in applications of laser printers and digital copiers that allow for full-color image formation. With background like this, as functions required for photoconductors, it is important to achieve both high-quality image and high-resistance.
For electrophotographic photoconductors used for laser printers and digital copiers using an electrophotographic process, typically, organic photoconductors (OPC) using organic photosensitive materials are widely used from the perspective of cost, productivity, and pollution-free properties. Layer configurations of organic photoconductors are classified broadly into a single layer configuration and a functional separation type laminate configuration. A PVK-TNF charge-transfer complex photoconductor that was an organic photoconductor put into practical use for the first time was a single layer photoconductor. In the meanwhile, a photoconductor using a PVK/a-Se laminate was reported by Hayashi and by Regensburger, in 1968, individually. Thereafter, a laminate type photoconductor having at least an organic pigment dispersion layer, an organic low-molecular-weight dispersion polymer layer, and a photosensitive layer composed of an organic material was reported by Melz et al. in 1977 and by Scchlosser in 1978. Each of these laminate-type photoconductors has a charge generating layer (CGL) that absorbs light to generate a charge, and a charge transporting layer (CTL) that introduces and transports the charge generated at the CGL thereto to neutralize the charge residing on the surface of the photoconductor, and is called as a functional separation laminate type photoconductor. Since such a functional separation type laminate photoconductor is more excellent in photosensitivity and durability than a single layer photoconductor and allows for individual molecular designs of a charge generating layer (CGM) and a charge transporting layer (CTM), it allows for wide selection of materials. For the reasons, the layer configuration of a functional separation laminate type photoconductor is now a primary layer configuration of organic photoconductors.
In the mechanism of formation of a latent electrostatic image in the functional separation type laminate photoconductor, the photoconductor is charged and irradiated with a laser beam, the laser beam passes through a charge transporting layer and is absorbed by a charge generating material contained in a charge generating layer to generate a charge. Then, the generated charge is introduced to the charge transporting layer at a boundary surface between the charge generating layer and the charge transporting layer and moves into the charge transporting layer by effect of electric field to negate the effect of the charge residing on the photoconductor surface, thereby a latent electrostatic image can be formed.
An organic photoconductor (OPC) suffers from a large amount of film exfoliation due to repetitive use thereof, and when film exfoliation proceeds on the photoconductor, it causes a reduction in the electric charge potential and degradation of photosensitivity of the photoconductor, resulting in acceleration of contamination of the photoconductor surface such as scratches, degradation in image density, and degradation in quality of images. Therefore, there is a need to improve abrasion resistance of organic photoconductors. Recently, importance is further placed on making an organic photoconductor have high-durability with increased high-speed performance or down-sizing of image forming apparatuses.
As methods to improve abrasion resistance of an organic photoconductor, there are known methods of imparting lubricating property to a photosensitive layer, hardening a photosensitive layer, or adding a filler to a photosensitive layer, and a method of using a polymer charge-transporting material instead of a low-molecular weight charge-transporting material (CTM). Recently, a method of forming a three-dimensionally crosslinked hardened film as a protective layer is also proposed. For example, Japanese Patent Application Laid-Open (JP-A) No. 5-66598 disclosed an organic photoconductor in which an acrylic resin hardened film is formed as a protective layer, and excellent abrasion resistance is achieved.
However, when film exfoliation of a photoconductor is prevented by these methods, ozone, NOx, or other oxidized substances generated in ambient surroundings are absorbed to the photoconductor surface, which may cause low-resistance of the outermost surface thereof to thereby cause problems with occurrences of image deletion such as image blur and reductions in dot-resolution. Conventionally, an image-blur causing material is exfoliated little by little together with a photosensitive layer and such a problem has been avoided to a certain degree. However, to respond to requests for giving further higher-resolution and higher durability to photoconductors in recent years, it is desired to develop a new means.
Then, as a method of alleviating affect of an image-blur causing material, a method of mounting a heater on a photoconductor is exemplified, however, such a method poses an impediment to downsizing of apparatuses and reductions in electric power consumption. A method of adding additives such as antioxidant to a photosensitive layer is also employed, however, addition of a large amount of additives which have no photoconductivity such as antioxidant to a photosensitive layer has negative influences on electrophotographic properties, such as low-photosensitivity and increases in residual electric potential.
Further, as a method of preventing occurrences of image blur or reductions in dot-resolution, for example, a method of adding a tertiary amine compound having a specific alkyl amine structure to a photosensitive layer is proposed (see Japanese Patent Application Laid-Open (JP-A) No. 2004-102080). However, when a tertiary amine compound is added to a crosslinked hardened film as disclosed in the proposal, it causes problems such as degradation of abrasion resistance and degradation of uniform film-forming property.
As described above, an electrophotographic photoconductor involving less film exfoliation owing to giving high-abrasion resistance to the photoconductor or process designing of peripherals of the photoconductor makes it impossible to avoid influences as side-effects thereof on quality of images such as occurrences of image blur and reductions in resolution, and it is difficult to achieve both high-durability and high-quality image. An electrophotographic photoconductor having a higher electric resistance is effective to prevent occurrences of image blur. In contrast, an electrophotographic photoconductor having a lower resistance is effective to prevent increases in residual electric potential. Because of the above-noted trade-off relation between these requirements, it is further difficult to solve the problems.